The X-Staticmap-API-Warning header says Too many geocoded markers requested (max is 15) and the docs clarify:
The request specifies more than the allowed maximum number of markers with a human readable address. The maximum is 15. Note that this limit applies only to markers specified as a human-readable address that requires geocoding. It does not apply to markers specified with latitude/longitude coordinates.
Seems like the solution is to use latitude and longitude to specify the marker locations instead of the city name.
maxbbraun
commented
4 years ago 
The
X-Staticmap-API-Warningheader saysToo many geocoded markers requested (max is 15)and the docs clarify:Seems like the solution is to use latitude and longitude to specify the marker locations instead of the city name.